A large number of reports on using, as a catalyst system capable of producing an olefin polymer, a catalyst system combining a metallocene compound which is a transition metal compound, such as zirconocene, with an organoaluminum oxy compound (also referred to as “aluminoxane” hereinafter) or a catalyst system combining a metallocene compound with a compound which reacts with a metallocene compound to form an ion pair (also referred to as an “ionizing ionic compound” hereinafter), such as N,N-dimethylanilinium tetrakis(pentafluorophenyl)borate, have been already made.
Since the ionizing ionic compound has low solubility in aliphatic hydrocarbon solvents, it is usually used by dissolving it in aromatic hydrocarbon solvents such as toluene. However, aromatic hydrocarbon solvents are generally high-boiling, and therefore, there is a possibility of remaining of them in an olefin polymer obtained by polymerization. It is preferable that no aromatic hydrocarbon solvent remains in the olefin polymer from the viewpoints of odor and toxicity, but if a step of separating/removing the solvent is introduced into the production process, the cost is sometimes increased. In recent years, therefore, change of solvents for dissolving ionizing ionic compounds from aromatic hydrocarbon solvents to saturated hydrocarbon solvents, such as aliphatic hydrocarbon solvents or alicyclic hydrocarbon solvents, has been needed.
As one attempt to meet this need, increase in solubility of ionizing ionic compounds has been thought. Specifically, a technique of introducing a bulky substituent into an ionizing ionic compound, that is, a technique of introducing a bulky substituent to the anionic molecule side of an ionizing ionic compound (see, for example, patent literatures 1 to 4) or a technique of introducing a bulky substituent to the cationic molecule side of an ionizing ionic compound (see, for example, patent literatures 5 and 6), has been proposed. Moreover, a technique of improving a structure of an ionizing ionic compound itself (see, for example, patent literature 7) has been also proposed. However, these techniques are not industrially advantageous because the structure of the ionizing ionic compound becomes complicated.